This invention relates generally to electrophotographic printing machines, and more particularly concerns an apparatus for automatically conditioning fused toner image carrying copy sheets to prevent sheet curl.
In a typical multicolor electrophotography, it is desirable to use an architecture which comprises a plurality of image forming stations. One example of the plural image forming station Architecture utilizes an image-on-image (IOI) system in which the photoreceptive member is recharged, re-imaged and developed for each color separation. This charging, imaging, developing and recharging, re-imaging and developing, all followed by transfer to paper, is done in a single to revolution of the photoreceptor in so-called single pass machines, while multi-pass architectures form each color separation with a single charge, image and develop, with separate transfer operations for each color. The single pass architecture offers a potential for high throughput.
In order to fix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to apply pressure and elevate the temperature of the toner to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of the support medium (typically paper). Thereafter, as the toner material cools, solidification of the toner material occurs, causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrostatographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fixing of electroscopic toner images onto a support has been to pass the support bearing the toner images between a pair of opposed roller members, at least one of which is internally heated. During operation of a fixing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls and thereby heated under pressure. A large quantity of heat is applied to the toner and the copy sheet bearing the toner image. This heat evaporates much of the moisture contained in the sheet. The quantity of heat applied to the front and back sides of the sheet are often not equal. This causes different moisture evaporation from the two sides of the sheet and contributes to sheet curling.
Paper curl is defined as any deviation from it's flat state. In the xerographic process, fusing drives moisture out. When regaining moisture, paper experiences curl due to differential hygro-expansitivity and thermo-expansivity between the paper and toner, and dimensional instability of paper due to its moisture history. The paper expands due to moisture re-absorption, but the toner does not expand, thus developing curl. Paper curl is one of the primary causes for paper handling problems in copying machines. Problems, such as, stubbing, image deletions and improper stacking result from copy sheet curl. These problems are more severe for color copies than black and white due to differences in their toner mass area, substrates, and fuser characteristics.
The following disclosures may be relevant to various aspects of the present invention. U.S. Pat. No. 5,264,899 describes a system for adding moisture to a copy sheet. The toner fixation step of electrostatographic reproduction desiccates paper, which may lead to the formation of a wave along the sheet edge. The invention uses a pair of porous rolls defining a nip to transfer additional moisture to the copy sheet as it is passed through the nip. The added moisture prevents edge wave formation.
U.S. Pat. No. 5,434,029 describes an apparatus and method of preventing the curling of a substrate having toner images electrostatically adhered thereto which substrate has been subjected to heat for the purpose of fixing the toner images to the substrate. Simultaneous constraint of the copy substrate and the application of moisture thereto is effected by passing the substrate through the nip formed by two pressure engaged rollers, one of which is utilized for applying the water to the back side of the substrate as the substrate passes through the aforementioned nip.
There remains a need for a system for preventing curl caused by the loss of moisture from a copy sheet during the fixing step of electrostatographic reproduction or printing that is practical for use with electrostatographic machines and is non-subjective in indicating when the correct water thickness is obtained on metering rolls. Ordinarily, the film thickness on a transfer roll is set-up by adjusting the interference between a rubber metering roll and the transfer roll and observing the sheen on the metering roll. When the correct water thickness is obtained, the surface appearance on the black rubber metering roll changes from a gloss to a matte-like finish. Currently, the only way to make this adjustment is by eye, i.e., observe the appearance of the metering roller. This procedure is not acceptable for customer machine set-up or in manufacturing.